Over the past 20 years increasing numbers of patients have undergone cardiopulmonary bypass for correction of congenital heart defects and or palliation of single ventricle type anatomy. While technical advances have been made in these surgical procedures, progress has been limited with regard to pharmacologic management of infants during the postoperative period. Although rapid extubation strategies have been applied at many centers in older infants and children, neonates and young infants still demonstrate morbidity after surgery, often resulting in prolonged time on mechanical ventilation. It was reported by the applicant that the recent Triiodothyronine Supplementation in Infants and Children Undergoing Cardiopulmonary Bypass (TRICC) clinical trial demonstrated that triiodothyronine supplementation with Triostat (liothyronine) reduced time on the ventilator after surgical procedures using cardiopulmonary bypass in infants under 5 months of age. However, the study design did not provide data in the format required by the Food and Drug Administration (FDA) for an indication change on the label for this drug. To apply for the labeling change for triiodothyronine (liothyronine) and significantly impact the current standard of care, the investigators aim to prospectively study the specific age groups. In the proposed study the hypothesis that triiodothyronine supplementation after cardiopulmonary bypass (CPB) in infants improves clinical outcome will be tested. This Phase 3, randomized, double-blind, placebo-controlled trial aims to obtain results confirming those of the TRICC trial that triiodothyronine supplementation with Triostat (liothyronine) in infants under 5 months of age reduces time on the ventilator after surgical procedures using cardiopulmonary bypass and confirming that triiodothyronine supplementation is safe in this age group after cardiopulmonary bypass.